1. Field of the Invention
The present invention relates to electro-optical scanners for reading a one or two-dimensional bar code symbology, and more particularly, to a portable or fixed position scanner capable of high resolution imaging in very low contrast symbology environments.
2. Description of Related Art
Optical imaging systems are commonly used to decipher data symbols printed on objects in order to identify the objects. A bar code symbol represents a common one-dimensional form of symbology, and comprises a pattern of vertical bars of various widths separated by spaces of various widths. Since the bar and space elements have different light reflecting characteristics, a reader can convert the symbology into an electrical signal by analyzing the light reflected from the symbol. The electrical signal can then be decoded to provide an alphanumeric representation of the symbol which identifies the object. Bar code symbols of this nature are now in common usage in various applications, such as inventory control, point of sale identification, or logistical tracking systems.
Since the conventional one-dimensional symbology requires a relatively large amount of space to convey a correspondingly small amount of data, so-called two-dimensional bar code symbologies have been developed. A two-dimensional symbology may comprise a matrix that occupies a uniform amount of space having a generally rectangular or square shape. Instead of bars and spaces, round or square characters disposed at particular rows and columns of the matrix correspond to the information being conveyed. As a result, a two-dimensional matrix symbology can compress significantly more data into a given volume of space than a conventional one-dimensional bar code.
In one particular application of a two-dimensional symbology, a small symbol can be placed directly onto items having low surface area, such as electronic components. The two-dimensional symbol could be formed directly onto the ceramic or plastic package of the electronic components by laser etching or other precision machining process. Since a two-dimensional symbology can compress fifty or more characters of data within a relatively small dimensional space, the symbol can store a unique identifier code for the component, including such information as lot or batch number, model number and/or customer code. The symbols can be used to automate the manufacturing or testing processes, and may also enable manufacturers to protect against component theft or forgery.
As known in the art, the two-dimensional symbols are read by scanners that convert the symbols into pixel information, such as described in U.S. Pat. No. 4,988,852 issued to Krishnan. The pixel information is in turn deciphered into the alphanumeric information represented by the symbol. Such scanners often utilize charge-coupled device (CCD) technology to convert optical information from the symbol into an electrical signal representation of the matrix. A light source illuminates the symbol, and light reflected off the symbol is focused onto the surface of the CCD device. The two-dimensional scanners may be provided in a portable device so that they can be brought into close proximity with the item on which the symbol is placed, such as disclosed in U.S. Pat. No. 5,378,883 issued to Batterman et al. The scanner may also be provided in a fixed-position device that images items as they pass by, such as on a production line.
A significant drawback of such etched or machined symbols is that they have very low contrast and, as a result, are difficult to image. Since the symbol characters are formed by shallow cuts into the surface of a component, there is little color difference between the characters and the remaining uncut surface area of the component. To compound this problem, electronic components often have a dull black finish that tends to further obscure the symbol characters. The characters can only be distinguished by the slight difference in shade due to shadows which form in the etched regions, and are often best imaged from an angle. U.S. Pat. No. 5,393,967 issued to Rice et al. discloses a system for reading symbols encoded as a low contrast relief pattern that projects a line of light at a first angle and views the projected light at a second angle. The line of light is swept or scanned across a surface containing the relief pattern, thus requiring relatively complex mechanical or optical systems.
Accordingly, a need exists for a method and apparatus for making high resolution images of very low contrast two-dimensional symbols.